This invention relates in general to fluid control valves and in particular to an improved structure for providing tactile feedback to an operator prior to actuation of a detent mechanism in such a fluid control valve.
In many hydraulic and pneumatic systems, control valves are provided for regulating the flow of fluid from a pressurized source to one or more controlled devices. Fluid control valves of this type generally include a case having a plurality of ports formed therein. A pressure port is provided which communicates with the pressurized source, while a tank port is provided which communicates with a fluid reservoir. One or more work ports are also provided which communicate with respective controlled devices. By selectively providing communication between the various ports, the operation of the controlled devices can be regulated in a desired manner.
For each of the work ports, a plunger valve assembly is typically provided within the case of the fluid control valve. Each of the plunger valve assemblies is operable to selectively provide communication between its associated work port and each of the pressure and tank ports. This is usually accomplished by means of an axially movable spool contained within the plunger valve assembly. The spool is movable upwardly and downwardly between opened and closed positions. In the opened position, the spool permits communication between the associated work port and the pressure port, thereby causing actuation of the controlled device. In the closed position, the spool provides communication between the associated work port and the tank port, thereby preventing actuation of the controlled device.
Axial movement of the spools is usually accomplished by means of a pivotable lever or joystick which is mounted on the upper end of the case. The lever is connected through respective linkages to each of the plunger valve assemblies. The lever is usually biased toward a center position. Pivoting movement of the lever in a first direction from the center position causes downward movement of the selected one of the spools from the closed position to the opened position. Similarly, pivoting movement of the lever in various other directions from the center position causes downward movement of the various other spools from the closed position to the opened position. The spools are usually biased upwardly by respective return springs toward the closed positions. These return springs typically react between spring seats formed on the case and portions of the associated linkages. As a result, an affirmative effort is required to pivot the lever from the center position so as to move the spools from their closed positions to their opened positions.
Fluid control valves of this type typically include a mechanism whereby the lever can be pivoted within a limited range of movement from the center position without opening either of the plunger valve assemblies. The purpose of this "dead band" range of movement is to prevent small movements of the lever from causing unintended movements of the spools and, therefore, operation of the controlled devices. To accomplish this "dead band" operation, a spring is usually provided in the linkage between the lever and each of the spools of the plunger valves. Once the lever has been pivoted beyond the end of the "dead band" range, the spool is moved from the closed position to the opened position. When this occurs, there is a step increase in the magnitude of the fluid pressure supplied to the controlled device, from zero pressure to a predetermined initial step pressure. Further pivoting movement of the lever causes a generally linear increase in the magnitude of the fluid pressure supplied to the controlled device from the initial step pressure to the maximum available system pressure.
In fluid control valves of this type, it is often desirable to provide a mechanism for temporarily retaining the lever in position once it has been pivoted to the point that the maximum available system pressure is being supplied to the controlled device. Such a detent mechanism thus allows the operator to release the lever while maintaining the supply of fluid pressure to the controlled device. The detent mechanism retains the lever in position until the operator applies a force to the lever which is sufficient to disengage the detent mechanism, or until the retaining force applied by the detent mechanism is otherwise released.
One common type of detent mechanism is known as an electromagnetic detent. Such a mechanism typically includes one or more armatures mounted to the base of the lever for pivoting movement therewith. Each of the armatures is aligned with an associated one of the plunger valve assemblies. One or more associated electromagnet assemblies are mounted on the control valve case so that pivotal movement of the lever brings one of the armatures into engagement with the associated electromagnet assembly. The magnetic force holds the armature to the electromagnet assembly until the operator manually pivots the lever to disengage the armature. Alternatively, a switch may be opened, for example by a predetermined movement of the controlled device, to de-energize the electromagnet assembly and release the armature.
In certain applications, it may be desirable to provide a supply of fluid pressure to the selected controlled device without actuating the detent mechanism. However, it is has been found to be difficult for an operator to accurately sense the point at which the detent mechanism will be actuated. Accordingly, a pre-detent tactile feedback mechanism has been used which provides a "feel" force to the lever just before the detent mechanism is engaged. Upon sensing this "feel" force, the operator is able to avoid further pivotal movement of the lever and the resulting actuation of the detent mechanism.
A number of such pre-detent tactile feedback mechanisms are known. In one such mechanism, a pair of ball bearings are disposed within a transverse cylindrical cavity in the plunger of the plunger valve assembly. A spring is also disposed within the cylindrical cavity between the two ball bearings to urge each of the ball bearings outwardly. As the plunger is moved axially within a bore in the case, the ball bearings roll in associated, longitudinally extending races formed in the bore. At the point at which the pre-detent "feel" force is desired, each of the races is provided with an inwardly extending projection. Thus, before the plunger can be moved axially further, the spring must be compressed to allow both of the ball bearings to move inwardly. This requires the application of additional force to the lever capable of being sensed by the operator. While such a pre-detent tactile feedback mechanism has been effective, it requires several additional parts and additional precision machining of the plunger and bore. In addition, it is difficult to disassemble the mechanism for maintenance.
In another type of pre-detent tactile feedback mechanism, a compression spring is disposed axially within the plunger bore. At the point at which the pre-detent "feel" force is desired, a specially threaded portion of the plunger contacts the spring. Additional axial movement of the plunger further compresses the spring, linearly increasing the force required to pivotally move the lever to the point where the maximum available system pressure is supplied to the controlled device. This also increases the force acting against the electro-magnetic detent used in the assembly. In addition, such a mechanism requires numerous special parts and is therefore relatively expensive.
It would therefore be desirable to provide an improved structure for a pre-detent tactile feedback mechanism for a fluid control valve which requires few parts and which is easy to assemble and disassemble. It would further be desirable to provide such an improved structure which effectively increases the force required to move the control lever just prior to engagement of the detent, but which does not significantly increase the force acting against the detent upon its actuation.